The present invention relates to a process for depositing silicon dioxide films, and more particularly to an improved process for depositing silicon dioxide films on the surface of substrates by bringing the substrates into contact with an aqueous solution of hydrosilicofluoric acid supersaturated with silicon dioxide.
It is widely carried out these days to coat the surfaces of various materials with silicon dioxide films (silica films). For example, it has been practiced for many years to reduce the reflection of a surface, e.g. a glass surface, by applying an alternating multi-layered coating of titanium oxide films and silicon dioxide films on the surface. It is also practiced extensively to form a silicon dioxide coating as a protecting film on the surface of a metal or alloy. Besides, when an alkali metal-containing glass such as soda lime glass or borosilicate glass is used as liquid crystal display panel or solar battery substrate glass, the surface of such a glass is coated with silicon dioxide in order to avoid the elution of its alkali metal component or components. Prevention of the elution of the alkali metal component or components from glass has become a technique particularly indispensable for maintaining long a service life of liquid crystal display devices or solar batteries.
A variety of techniques has heretofore been employed to form silicon dioxide films on glass surfaces, including the vacuum deposition, sputtering, CVD and dip coating (dipping) techniques. These conventional techniques are, however, accompanied by such drawbacks that the formation of silicon dioxide films is costly as these conventional techniques require expensive facilities and incidental facilities and, moreover, they can be applied to small substrates only.
In contrast to the above-mentioned techniques, there is known a method of depositing silicon dioxide films on the surface of a substrate, which can be effected with simple facilities and is capable of depositing the silicon dioxide film on a large substrate, and in which the silicon dioxide film is formed by immersing the substrate into a treating solution prepared by adding a reagent such as boric acid to an aqueous solution of hydrosilicofluoric acid (hexafluorosilicic acid) substantially saturated with silicon dioxide, as disclosed, for example, in U.S. Pat. No. 2,505,629 and Japanese Patent Publication Kokai No. 60-33233. This method will hereinafter be referred to as "liquid phase deposition method".
The principle of depositing a silicon dioxide film by the liquid phase deposition method is that the treating solution which is an aqueous solution of hydrosilicofluoric acid substantially saturated with silicon dioxide is in the following equilibrium state: EQU H.sub.2 SiF.sub.6 +2H.sub.2 O.revreaction.SiO.sub.2 +6HF (1)
and when boric acid is added to this treating solution, hydroborofluoric acid is then produced from boric acid and hydrofluoric acid in accordance with the following reaction: EQU H.sub.3 BO.sub.3 +4HF.fwdarw.HBF.sub.4 +3H.sub.2 O (2)
and, as a result of the consumption of hydrofluoric acid, the equilibrium in formula (1) proceeds to the right-hand side, and the amount of SiO.sub.2 in the treating solution is increased to a supersaturation level, followed by the deposition of a film of SiO.sub.2 on the surface of the substrate immersed in the treating solution.
The above deposition method has the advantages that the formation of the film is possible at a low temperature, and that it is possible to form films on substrates of any materials and any shapes, but it has the disadvantages that it is difficult to reuse the used treating solution because of inclusion of boric acid and that a complicated procedure is required for detoxifying treatment of waste liquid in disposal of the used treating solution.
In a fluorine-containing waste liquid, the fluorine is generally separated and removed as a CaF.sub.2 precipitate formed by the addition of Ca(OH).sub.2. However, the fluorine concentration in the waste liquid produced in the conventional liquid phase deposition method cannot readily be decreased by single addition of Ca(OH).sub.2, and it is therefore necessary to repeat many times the cylcle of the addition of Ca(OH).sub.2, precipitation and separation.
Further, the conventional liquid phase deposition method has the disadvantage that the formed silicon dioxide films are contaminated with impurities, e.g. boron resulting from boric acid used for forming the supersaturated solution.
Accordingly, it is an object of the present invention to provide a process for depositing silicon dioxide films on substrates, wherein an apparatus used for depositing the silicon dioxide films is simple, the silicon dioxide films can be formed on a larger substrate, and a procedure required for detoxifying treatment of the waste liquid in disposal of the used treating solution is not complicated.
A further object of the present invention is to provide a process for forming a dense film of silicon dioxide having a high purity on substrates in a shortened period of time.
Another object of the present invention is to provide a process for depositing silicon dioxide films on substrates wherein the used treating solution is reused.
These and other objects of the present invention will become apparent from the description hereinafter.